During anti-lock controlled braking and acceleration traction control operating modes of a vehicle, it is desirable to maintain a close correspondence between steering wheel angle and vehicle steering. However, depending upon the characteristics of the vehicle, the vehicle may tend to have an increased understeer, so the operator must turn the steering wheel more to get the vehicle to properly turn. The vehicle may also oversteer, requiring the operator to turn the steering wheel less for the vehicle to properly turn. When steering a vehicle, the speed of the driven or braked wheels of the vehicle can effect vehicle steering performance to deviate from its preset understeer characteristics.
When a vehicle turns, the wheels closest to the center of the turning radius are referred to as the inner wheels and the wheels farthest from the center of the turning radius are referred to as the outer wheels. Referring to FIG. 1, a vehicle 12 has four wheels 14, 16, 18, and 20, a wheel base L, and each wheel has a distance D from the centerline 21 of the vehicle 12. The vehicle is shown with the front wheels 16 and 20 turned so that the vehicle makes a right turn such that the center of gravity 22 of the vehicle 12 turns at a radius R around the center 24 of the turn. In this example, the wheels 18 and 20 are inside wheels and the wheels 14 and 16 are the outside wheels. When the vehicle turns left, wheels 14 and 16 are the inside wheels and the wheels 18 and 20 are the outside wheels.
An operator negotiates a turn which has a desired turning radius of R.sub.d by rotating the vehicle steering wheel. However, if vehicle understeer characteristics (note: "understeer characteristics" may infer both understeer and oversteer depending upon the sign) deviate from the vehicle's nominal setting, the actual turning radius of the vehicle, R, varies from the desired turning radius, R.sub.d. Vehicle understeer occurs when the steering wheel is turned a specific amount, indicating a desired steering radius R.sub.d, but the actual steering radius, R, is greater than R.sub.d. Vehicle understeer is characterized by an understeer coefficient K.sub.u, which is a function of the static normal load on the front and rear tires and the cornering stiffness of the tires. An understeer coefficient K.sub.u greater than zero occurs during vehicle understeer, and is considered stable, however, it is not desirable to have a very large understeer coefficient.
Vehicle oversteering occurs when the steering wheel is turned to a specific amount, indicating a desired steering radius R.sub.d, but the actual steering radius, R, is less than R.sub.d. This occurs when the understeer coefficient K.sub.u is less than zero.
Because the cornering stiffness of a given tire varies with a number of operational parameters including inflation pressure, normal load, braking effort, and lateral force, the understeer coefficient K.sub.u varies with the operating conditions of the vehicle. Therefore, it is important to maintain the effective understeer coefficient within a certain desired level, 0.ltoreq.K.sub.u .ltoreq.K.sub.d, to provide stable and consistent vehicle behavior, allowing the driver to direct the vehicle without excessive steering.
It is further known that while turning, the lateral force on a vehicle wheel can be increased to enhance directional control by decreasing wheel slip to levels below the slip resulting in a maximum longitudinal force.